Update contrib guide

[shoeffner]

(Note that this PR already refers to changes which are included in #205 ).

I updated the contrib guide as follows:

Replace duplicate refs with anonymous refs

For example:

`Abc <https://example.org/abc.html>`_
`Abc <https://abc.example.org/>`_

Would cause the rst parser to complain about duplicate refs. Replacing _ with __ makes the ref anonymous, i.e., Abc will no longer be treated as the ref name.

`Abc <https://example.org/abc.html>`__
`Abc <https://abc.example.org/>`__

Include CONTRIBUTING.rst properly

While building the docs with tox -e docs, ../CONTRIBUTING.rst was missing. After fixing this, there was no link to the Contributor Guide due to a missing title. So I added the title to contributing/contributing.rst and included CONTRIBUTING.rst from line 3 to prevent the double title. Finally, the language had to be set to non-None to avoid another WARNING.

Update the contrib guide

• I updated refs to develop/master to use main.
• I changed the setup guide to refer to poetry and tox, and explained a little bit more how to run the tests / setup the test environment (including .env and handling environment variables for poetry/tox)
• I updated how to build the docs with tox via poetry and how to serve them with a local browser
• I added a few notes on the docker compose setup and some hints on possible pitfalls I encountered when attempting to debug the tests.

Closes #193 .